The luminescent bodies of organic electroluminescent elements (hereinafter may be referred to as “organic EL elements”) can be formed in a planar shape, and the color of light emitted therefrom can be white or a color close to white. Therefore, it is contemplated that such organic EL elements are used as light sources of lighting devices for illuminating areas in living environments etc. or in applications for backlights of display devices.
As an example of the organic EL elements used in lighting applications, white organic EL elements are being manufactured. Many of such white elements are of the stacked or tandem type in which luminescent layers that generate light with complimentary colors are stacked. The stacks of such luminescent layers are mainly stacks of yellow/blue luminescent layers or green/blue/red luminescent layers.
However, currently known organic EL elements are unsatisfactory in terms of efficacy for the above-described lighting applications. Therefore, there is a necessity for improving the light extraction efficiency of such organic EL elements.
One known method for improving the light extraction efficiency of organic EL elements is to provide various structures on their light-extraction substrates. For example, it is proposed to provide prisms containing a fluorescent compound on the light-emitting surface of a light source device (Patent Document 1). It is also proposed to provide a micro-lens array on the light-emitting surface of a light source device (Patent Document 2). With these structures, light can be gathered in a favorable manner, and the efficiency is thereby improved.
However, when any of these structures is adopted for the above-described lighting organic EL element of the stacked type and a concave-convex structure such as prisms is provided on the light-emitting surface of the light source device, the apex portions of the concave-convex structure are easily broken off. Therefore such a structure causes difficulty in increasing the mechanical strength of the device.
One possible measure to improve the mechanical strength of the device is to employ, as the concave-convex structure, a structure including a plurality of concave portions having oblique surfaces and flat portions disposed around the concave portions. With such a structure, the mechanical strength of the light-emitting surface can be improved because of the structure of the flat portions.
However, with the aforementioned structure, even a slight insufficiency in accuracy of, e.g., the height of the flat portions may cause interference of reflected light due to the slight differences in height of the flat portions. As shown in FIG. 15, such interference may cause a rainbow unevenness observed on an reflected image on a screen. Such a rainbow unevenness significantly reduces the quality of the light source. Although such a rainbow unevenness may be eliminated by extremely increasing the accuracy of the mold for forming the concave-convex structure, mass production with such high accuracy is difficult and causes an increase in manufacturing cost.